As is known, a PEM fuel cell has catalyst layers on the cathode and anode side of the membrane, possibly with optional diffusion layers formed as a coating on the cathode and anode substrates, respectively, which are on the non-membrane sides of the cathode and anode catalysts. The substrates, also known as carbon papers, are highly porous, which may, for instance, have on the order of 70% porosity with pores on the order of 30 microns in diameter. In order to prevent the oxidant reactant gas (such as air) and the fuel reactant gas (such as a hydrogen containing gas) from escaping through the side edges (those edges not in communication with respective external manifolds) it has been known to provide edge seals to the anode and cathode substrates.
In FIG. 1, an exemplary PEM fuel cell 9, of the general type known to the prior art, includes a proton exchange membrane 11, an anode catalyst 12 which may comprise a coating on the membrane of on the order of 10 micron thickness, an anode substrate 14, which may, optionally, have an anode diffusion layer comprising a coating 15 on the order of 25 microns on the surface of the substrate 14. The fuel flow field may typically comprise an anode flow field water transport plate 18 having fuel reactant gas flow channels 19 therein and a degree of porosity to permit water, typically from a coolant flow channel (not shown), to be absorbed in the fuel reactant gas so as to provide moisture through the anode layers to the membrane 11. Similarly, on the cathode side, there is a cathode catalyst 22, there may be an optional cathode diffusion layer 23, a cathode substrate 26, and an oxidant reactant gas flow field, typically comprised of a cathode flow field water transport plate 27 having oxidant reactant gas flow field channels 28 therein. In FIG. 1, external fuel reactant gas manifolds (not shown) will be in fluid communication with the fuel reactant gas flow field channels 19, and external oxidant reactant gas manifolds (not shown) will be in fluid communication with the oxidant reactant gas flow field channels 28.
To prevent gases from leaking from the substrate layers 14, 26, it is common to employ an edge seal 31, 32 which consists of a thermoplastic film such as polyvinylidene chloride (KYNAR®), or an elastomer, such as a silicone rubber, extruded into the substrate. Plastic films 33 may be provided as fillers at the edges of the anode and cathode catalysts and optional diffusion layers.
Interfacial seals between the anode water transport plate 18 and the anode substrate 14, as well as between the cathode water transport plate 27 and the cathode substrate 26 may typically comprise silicone rubber closed cell foam gaskets 35, 36, respectively, held in place, prior to compression in making the fuel cell stack, by pressure sensitive adhesive 37, 38. The pressure sensitive adhesive may be an acrylic adhesive or a silicone adhesive. It has been found that if an acrylic adhesive is used, the life of the fuel cell is limited to between 2,000 hours and 5,000 hours because of gas leakage due to the corrosion of the adhesive. A problem with either adhesive is the extra steps required to produce a unitized electrode assembly 40 (including the catalyst coated membrane and the substrates (with or without diffusion layers), and the extra steps required to produce the flow field seals assembly.
As illustrated in FIG. 2, the prior art process requires a hot lamination step 43 to provide the anode substrate 14 with the impregnated seal 31. A similar hot lamination step 44 is required to provide the cathode substrate 26 with the impregnated seal 32. Then, a third hot lamination step 45 is required to join the substrates 14, 26 with the catalyst coated membrane 11, 12, 22, in order to produce the unitized electrode assembly 40. The flow field plates 18, 27 have their respective silicone rubber gaskets 35, 36 adhered to them by pressure sensitive adhesive 37, 38 in compression steps 48, 47 to provide fuel and oxidant flow field plates with seals 48, 49. These are then brought together, along with other, similar fuel cell components to form a fuel cell stack assembly 50.
Fuel cells of the type described with respect to FIG. 1 are illustrated in U.S. Pat. Nos. 6,020,083, 6,159,628, and 6,187,466.